It is particularly difficult to apply solder coatings with consistent thicknesses and configurations to circuit boards having conductive lands with close edge-to-edge spacings (e.g. 20 mils or less). Solder screening is not satisfactory as non-uniform amounts of solder often remain after the screen is removed. When solder is reflowed to attach a chip to the lands, a "solder bridge" can occur between adjacent lands causing a potentially catastrophic failure. It has been found increasingly difficult to use a screening process where solder having a thickness of less than 0.002 inches is either desirable or required. These thicknesses are employed for attaching tape automated bonding (TAB) packages to circuit boards and insure that no solder bridging occurs.
TAB packaging is seeing ever wider usage as it can be handled in an automated fashion and is one of the least expensive packaging configurations. It is important therefore, that even with the stringent dimensional requirements placed upon the soldering systems by TAB packages, that the cost of solder application be minimized.
The prior art has attempted to solve the precision solder application problem via a number of routes. U.S. Pat. No. 4,209,893 to Dyce et al provides a solder carrier strip wherein the strip is punched and solder preforms are pressed into the punched areas. This technique, while usable for relatively large circuit configurations, is not usable for the miniature TAB packages for the reason that mechanical punching is too imprecise.
U.S. Pat. Nos. 4,354,629 and 4,484,704 to Grassauer et al both describe a system for joining flat or ribbon cables to connectors. Those patents teach the use of a pair of polymeric webs with a continuous solder preform packaged therebetween. One of the webs has a series of windows disposed in it. The windows are registered over conductors to be soldered, with pressure and heat being applied to cause solder reflow. The partitions between adjacent windows prevent solder bridging. Here again, this technique is suitable for relatively large size conductor/circuit board arrangements; however it is not appropriate for microminiature circuit configurations. For instance, the continuous layer of solder underneath the window divisions would tend, upon melting, to create solder bridging if closely spaced lands were being tinned.
Accordingly, it is an object of this invention to provide a precision solder transfer method particularly suitable to microminiature package configurations.
It is another object of this invention to provide a precision solder transfer technique which is adapted to employ manufacturing techniques commonly employed for circuit boards and semiconductor devices.